Horizontal double disc grinder with anti-vacuum control

ABSTRACT

A double disc abrading machine, such as a double disc grinder, which is particularly adapted for grinding opposite surfaces of thin workpieces (W), includes the injection of a metered amount of air into the coolant supplied through the spindles (13L and 13R) to eliminate a partial vacuum formed between the abrasive discs (14L and 14R). It is believed that hydrodynamic forces create a partial vacuum between the closely spaced abrasive discs (14L and 14R) which are rotated in the same or opposite directions. There may also be cohesive and/or adhesive forces acting to draw the abrasive discs (14L and 14R) too close together. It is believed that this combination of forces and the ambient air pressure exerted on the outside of the abrasive discs (14L and 14R) causes the workpieces (W) to be ground undersize. The metered air is injected into the coolant during the grinding process to eliminate the vacuum effect. In some instances it is desirable to add supplementary un-metered air at the completion of the grinding process so that the discs may be retracted in unison at a rapid rate.

United States Patent [191 Dunn [ Nov. 27, 1973 HORIZONTAL DOUBLE DISCGRINDER WITH ANTI-VACUUM CONTROL [75] Inventor:

[73] Assignee: Litton Industries, Inc., Beverly Hills,

Calif.

[22] Filed: Apr. 30, 1971 [21] Appl. No.: 138,871

Elman R. Dunn, Roscoe, Ill.

Primary ExaminerHarold D. Whitehead Attorney-Joseph R. Spalla [57ABSTRACT A double disc abrading machine, such as a double disc grinder,which is particularly adapted for grinding opposite surfaces of thinworkpieces (W), includes the injection of a metered amount of air intothe coolant supplied through the spindles (13L and 13R) to eliminate apartial vacuum formed between the abrasive discs (14L and 14R). It isbelieved that hydrodynamic forces create a partial vacuum between theclosely spaced abrasive discs (14L and 14R) which are rotated in thesame or opposite directions. There may also be cohesive and/0r adhesiveforces acting to draw the abrasive discs (14L and 14R) too closetogether. It is believed that this combination of forces and the ambientair pressure exerted on the outside of the abrasive discs (14L and MR)causes the workpieces (W) to be ground undersize. The metered air isinjected into the coolant during the grinding process to eliminate thevacuum effect' In some instances it is desirable to add supplementaryun-metered air at the completion of the grinding process so that thediscs may be retracted in unison at a rapid rate.

3 Claims, 4 Drawing Figures lOL Patented Nov. 27, 1973 3,774,348

2 Sheets-Sheet 1 INVENTOR $(LF 43 T T M ELMAN R. DUNN AIR SUPPLY 47 BY MATTORNEY Patented Nov. 27, 1973 2 Sheets-Sheet ILSA N 40R CARRIER INLIMIT 4TR3 I CYCLE START I I l IPB I SPARKOUT CLUTCH iE CYCLE sTART I amI I3 CA A fila LRRIER DVANCE 5CRl V IL II 4cRI 4TR2 FACECUT ADVANCE I%7CR2 AL 4TRl 3L8 4L5 I SPARKOUT MOTOR N N LH FACECUT RH FACECUT W m AIRINJECTOR GTRI m AIR INJECTOR 50L 0 cARRIER ADVANCE II b Y sou) FACECUTADVANCE H L Z sou AIR INJECTOR II L 4TR SPARKOUT CLUTCH REsET TIME 4TRIO X 0 4TR2 X X 0 4 4TR3 x x 0 MR4 O O X INVENTOR ELMAN R. DUNN ATTORNEYHORIZONTAL DOUBLE DISC GRINDER WITH ANTI-VACUUM CONTROL BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates ingeneral to new and useful improvements in double disc grinders, and moreparticularly, to a double disc grinder which is particularly adapted forgrinding the opposite surface portions of extremely thin workpieces. Ametered supply of air is directed into the coolant volume during thegrinding operation to eliminate the hydrodynamic forces which create apartial vacuum which interferes with the controlled movement of the discsupporting heads.

2. Description of the Prior Art An apparatus for advancing the opposeddiscs of a horizontal disc grinder is disclosed in U.S. Pat. No.3,001,337, issued Sept. 26, 1961. An improved advancing apparatus isdisclosed in the inventors pending application, Ser. No. 51,860, filedJuly 2, 1970 and now US. Pat. No. 3,721,046, dated Mar. 20, 1973. Thefeeding movement of the abrasive discs is synchronized to advance at anequal rate controlled by various hydraulic and mechanical arrangements.The power to effect the feeding movement is applied in equal amounts toeach of the disc supporting members by means of identical feedmechanisms. In the prior art, workpieces have been ground to apredetermined size with parallel sides, after which the abrasive discsare retracted. However, when the surface portions of extremely thinworkpieces are ground, a vacuum-like phenomenon occurs, and it isbelieved a partial vacuum is created between the abrasive discs. Thiscondition seems to be caused when the desired amount of coolant isdirected through hollow spindles into the grinding zone, and thecentrifugal or rotational forces acting on the coolant preventpenetration of atmospheric air therebetween when the abrasive discs arepositioned very close together. Therefore, the positive movement of theabrasive discs toward one another cannot be effectively controlled untilthe effect of static pressure of the atmosphere acting on theexterior'surfaces of the abrasive discs is overcome or satisfactorilycounterbalanced. Therefore, thin workpieces are 'sometimes groundundersize because the abrasive discs are ad vanced closer together thanintended by the feed mecha certain amount of entrained air, in practice,it has not However, these air nozzles are for an entirely differentpurpose and could only be used where the workpieces are relatively largein size. In such cases, there probably is no partial vacuum problem dueto the wide spacing of the discs. Furthermore,application of the airnear the periphery of the abrasive discs, as taught in U. S. Pat. No.3,151,422, is considered ineffective in solving the problem of a partialvacuum since the outward rotational movement of the coolant wouldprevent inward penetration of the air.

SUMMARY OF THE INVENTION In accordance with the invention, there isprovided a double disc abrading machine for abrading the parallel sidesof a thin workpiece to a precise dimension. The machine includes a pairof abrasive discs, which are supported and rotated by respectivespindles. Facilities are provided to advance and retract the discs, andfor supporting the workpiece. Coolant is supplied through at least oneof the spindles to the area between the discs. In order to eliminate thepartial vacuum that forms between the discs, a volume of air is directedinto one of the spindles during the abrading operation.

' In the preferred embodiment, the air which is injected during theabrading operation is metered. Furanism, due to the assisting effect ofnormal atmospheric forces acting on the exterior of the abrasive discs.Also, the tooling arm or work holder maybe damaged when the abrasivediscs are not retracted in unison, due to the vacuum or suction betweenthe discs. The work holder is susceptive to damage because of its thinsize and the fact that the clearance between the work holder and thesurface of the abrasive discs must be kept to a minimum to provideadequate support for the workpiece. This latter problem has beenpreviously solved by the inventor through the injection of a quantity ofair into the coolant system after the grinding cycle was finished, butbefore the abrasive discs were retracted.

In U. S. Pat. No. 2,089,040, issued Aug. 3, 1937, there is disclosed anapparatus for supplying coolant fluid to the central portion of therotating grinding discs. However, there is no suggestion that meteredquantities of air could be added to the coolant to overcome a partialvacuum created between the abrasive discs. While the coolant may haveinherently contained thermore, at the completion of the abradingoperation, but prior to the retraction of the discs for workpieceremoval, an additional amount of air is injected into one of thespindlesto assist in separating the closely spaced discs.

A primary object of the instant invention is to provide a pneumaticsystem of metered air for reducing or eliminating a partial vacuumformed between narrowly spaced abrasive discs to enable specificworkpiece size tolerances to be maintained.

Another object is to prolong the life of a thin tooling member whichhouses a workpiece in the grinding station, by avoiding overheating andbuckling.

Another objectis to prevent either of the grinding heads from driftinginwardly in response to the cohesive and adhesive forces of the coolantwhich forms a vacuum'between the abrasive discs, which later resistsretraction of the abrasive discs.

The above and other features and advantages of the present inventionwill become better understood from the detailed description of theinvention that follows, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a horizontaldouble disc grinding machine embodying the principles of the invention,showing the workpiece, workpiece fixture, grinding discs, and othersupporting structure;

FIG. 2 is a diagrammatic pneumatic and coolant circuit showing means foreliminating the hydrodynamic forces which create a vacuum-likephenomenon;

FIG. 3 is a wiring diagram showing the control circuit for operating theretraction of the grinding heads in unison; and

FIG. 4 is a diagrammatic schedule of the contact and status of thesparkout control device 4TR of FIG. 3.

DESCRlPTlON OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis illustrated a longitudinal feed mechanism L and 10R, which advancesan opposing pair of grinding heads 11L and 11R toward a thin workpieceW. EAch of the grinding heads 1 1L and 11R is slidably mounted on amachine bed 12. The grinding heads 11L and 11R include spindles 13L and13R, which carry abrasive discs 14L and 14R. The abrasive discs 14L and14R are driven through suitable connections by motors 161. and 16R.

The means for advancing the abrasive discs 14L and 14R toward oneanother consists of feed crews 17L and 17R mounted in the bed 12. Thefeed screws 17L and 17R are in operative engagement with feed nuts 18Land 18R, which are mounted in bed 12, and in alignment with the grindingheads 11L and 11R respectively.

The details of a preferred feed mechanism are disclosed in applicantsco-pending application Ser. No. 5l,860, filed July 2, 1970, which isexpressly incorporated herein. Accordingly, there will be describedhere, only the anti-vacuum control apparatus which cooperates with thefeed mechanism to enable the grinding heads 11L and 11R to be easilyretracted in unison, following the simultaneous removal of apredetermined amount of stock from the parallel faces or sides ,21 and22 of the workpiece W. It is to be understood that the invention hasutility with other feed mechanisms, and is not limited to the specificmechanism disclosed herein or in the above referenced pending patentapplication.

The workpiece W is held by a gun-type work carrier assembly 23 which isthe subject of applicants U. S. Pat. No. 3,503,155, issued Mar. 31,1970, which is expressly incorporated herein. The work carrier assembly23 is secured to the bed 12 and includes a work fixture or carrier 24having an opening 26, which retains the workpiece W in an accurateposition within the carrier 24. The carrier 24 advances the workpiece Wbetween the abrasive discs 14L and 14R by means of a fluid motor orcylinder 27. The cylinder 27 is mounted on top of a track unit 28 andincludes a piston rod 29 which is secured to the carrier 24. Movement ofthe piston rod 29 advances and retracts the carrier 24 and the workpieceW into the grinding position 30, as shown in FIG. 2, and the abrasivediscs 14L and 14R are advanced in unison. The carrier 24 is reciprocatedduring the grinding operation by movement of the piston rod 29 to effectthe desired finish of the workpiece W and to promote uniform disc wearin a conventional manner.

The desired amount of liquid coolant from a supply source is deliveredto the hollow spindles 13L and 13R by lines 31 and 32, respectively, forgrinding purposes only. Also, coolant is supplied through a line 33which is directed into the fixed hood 34, surrounding the abrasive discs14L and 14R, for flushing swarf out of the hood. The volume of coolantdirected through the hollow spindles 13L and 13R to the workpiece W mustbe sufficient to provide a cooling effect during the grinding operationand to remove metal and abrasive particles. When grinding thinworkpieces W, too little coolant permits overheating and distortion ofthe workpiece W, and too much coolant results in hydrodynamicdisturbanceof the thin workholding or guiding tooling, such as the carrier 24.

When the suitable amount of coolant is supplied to the workpiece W, avacuum-like phenomenon occurs between the abrasive discs 14L and 14R,and apparently a partial vacuum is created between the abrasive disc 14Land 14R. This condition is believed to be caused by the volume ofcoolant and by the centrifugal force of the coolant flowing from thespindles 13L and 13R, which displaces the air between the cuttingsurfaces of the abrasive discs 14L and 14R, and possibly by the cohesiveand adhesive forces of the coolant. The discs 14L and 14R appear to bedrawn together by suction as a result of the above forces.

It is important that each of the abrasive discs 14L and 14R be retractedin unison at completion of the grinding cycle after the parallel sidesof thin workpieces W have been ground, in order to maintain size, andbecause of the minimum clearance which exists between the sides of thecarrier 24 and the cutting surfaces of the abrasive discs 14L and 14R;

Actual tests have been conducted that demonstrate the magnitude of theproblem. Using 30 inch diameter discs 14L and 14R that were spaced 0.066of an inch apart, tests were performed both with and without the carrier24 and workpieces w positioned between the discs 14L and 14R. When thecarrier 24 and workpieces w were in position, and with the discs 14L andR rotating in opposite directions the grinding heads 11L and 11R movedtogether as'much as 0.0017 of an inch (left) and 0.0025 of an inch(right) when full coolant flow began. This represents a possible errorof 0.0042 of an inch in the thickness of a workpiece W which issignificant when grinding workpieces W to to]- erances, such as 1*:0.005 of an inch or less. When the discs 14L and 14R were rotated in thesame direction there was slightly less movement.

Other tests were performed without the carrier 24 in place, but with thediscs 14L and 14R again spaced 0.066 of an inch apart. With the discs14L and 14R rotating in opposite directions, the grinding heads 11L and11R moved together as much as 0.0032 of an inch (left) and 0.0035 of aninch (right) when full coolant flow began. This represents a possibleerror of 0.0067 of an inch in the thickness of a workpiece W. If thecoolant flow was reduced, to say one quarter of normal full flow, therewas also less movement of the grinding heads 11L and 11R. When thecoolant was shut off entirely and with the discs 14L and 14R rotating,there was only a negligible amount of movement, such as 0.0001 of aninch on each of the grinding heads 11L and 11R, which could beattributable to indicator error.

When the grinding heads 11L and 11R are thus actually shifted towardeach other, it should be apparent that there is substantial interferencewith the precise control by the feed system of the relative positions ofthe discs 14L and 14R. Precisely controlled relative positioning of thefaces of the abrasive discs 14L and 14R becomes exceedingly criticalwhen the prescribed tolerance range of a workpiece W is within closelimits.

It should be understood that the width of the carrier 24 mustbe justslightly less than the width of the finish ground dimension of theworkpiece W, because the carrier 24 remains between the cutting surfacesof the abrasive discs 14L and 14R during the entire grinding operation,as shown in FIG. 2. As an example, the

width of the carrier 24 may be 0.048 of an inch when a workpiece W. tobe ground has a finished dimension of 0.054 of an inch. This provides aclearance of 0.003 of an inch between each side of the carrier 24 andthe respective abrasive discs 14L and 14R. Also, the height of thecarrier 24 is sufficient to withstand the torque which is appliedthrough the workpiece W by each of the abrasive discs 14L and 14R, andto resist sidewise deflection, as much as is possible.

Deflection occurs when the hydrodynamic coolant pressure upon one sideof the carrier 24 exceeds the corresponding pressure on the oppositeside. The carrier 24 may then be deflected to contact one of therespective abrasive discs 14L or 14R, and overheating, buckling, andultimate destruction may result.

The carrier 24 can also be damaged if one of the abrasive discs 14L or14R drifts inwardly, during completion of the grinding cycle. This canoccur if the grinding heads 11L or 11R are urged toward each other, asthe ambient air pressure against the rear side of the abrasive discs 14Lor 14R acts upon them in the manner of huge pistons to cause inwardmovement.

It should be understood that this condition exists only when thinworkpieces are being ground. The abrasive discs 14L and 14R are normallyrotated'in opposite directions to reduce the torque load on the carrier24 when thin workpieces are being ground. However, the vacuum problembetween the abrasive discs 14L and 14R still exists when the abrasivediscs 14L and 14R are rotated in the same direction.

While tests have not been made to measure any movement of the grindingheads 1 1L and 1 1R when the instant invention is employed, actual testshave shown that the above problems are eliminated by employing thisinvention during a grinding operation.

The anti-vacuum control system shown in FIG. 2 includes a line 36 whichis connected from a shop air supply 37 through a filter 43 to a line 46having a metering valve 47 and a check valve 44. The metering valve 47provides a continuous metered amount of air to the spindle 13R through aconnection 41 of a rotary union 42. The check valve 44 prevents coolantfrom backflowing into the air control system. A control valve 38 ismounted in a line 39 which is connected between the lines 36 and 46. Thevalve 38 is used to supply an additional, unmetered amount of air to thespindle 13R before retraction at the completion of a grinding operationin order to facilitate the separation of the abrasive discs 14L and 14R.

It should be understood that in the preferred embodiment, theanti-vacuum system operates to inject a metered supply of air throughthe valve 47 throughout the entire grinding oepration to reduce oreliminate the partial vacuum formed in the area between the abrasivediscs 14L and 14R. This is particularly important during the finalportion of a grinding cycle, known as the sparkout" or dwell interval,when the abrasive discs 14L and 14R are neither being advanced norretracted by the feed mechanism L and 10R. The partial vacuum whichnormally occurs between the closely spaced abrasive discs 14L and 14R isthus prevented from forming as the metered inflow of air remainscontinuous whenever coolant is being directed through the lines 31 and32 to the spindles 13L and 13R. Inward movement of the abrasive discs14L and 14R can then be more precisely controlled by the feed mechanisms10L and 10R, and the abrasive discs 14L and 14R are more preciselyadvanced and stopped in unison without the undesirableforces resultingfrom the vacuum which interferes with such movement at completion of thesparkout operation positions.

As previously described, it is desirable that both of the abrasive discs14L and 14R be retracted immediately and simultaneously at theconclusion of the sparkout operation. Therefore, just prior to themoment of retraction, the additional unmetered volume of air is injectedinto the fluid body between the abrasive discs 14L and 14R through thevalve 38 which is at that moment, shifted to the right (FIG. 2) for abrief time interval. The sum effect of the two systems of air injectionoccurring during this interval prevents less-thanambient pressurefrornbieng generated between the closely spaced discs 14L and 14R by thesudden separation of the discs. Also, the injected air appears to reducethe tenacious quality of the fluid coolant body in rotative circulationbetween the discs 14L and 14R.

OPERATION motors 16L and 16R. Coolant is being directed from a supplysource to the hollow spindles 13L and 13R through the lines 31 and 32respectively, in a conventional manner, as shown in FIG. 2. Check valves48 and 49 are provided in lines 31 and 32 respectively to'preventbackflow of coolant. A constant volume of air is directed into thespindle 13R through the valve 47 in the line 46 to prevent the grindingheads 11L and 11R from drifting inwardly.

An unground workpiece W is placed into the opening 26 of the workcarrier 24 and a -cycle startpushbutton IPB (FIG. 3)- is depressed. Acircuit is completed through a -cycle stoppushbutton 2PB and a normallyclosed contact 4TR3, to energize a -cycle startrelay SCR, A sparkoutclutch 4TR is energized which closed a contact 4TR1, and a -carrieradvancerelay 6CR is energized through a limit switch contact lLSB whichis normally closed.

The energization of the relay SCR closes a contact 5CR'1 which providesa holding circuit around the pushbutton IPB, after its release, tomaintain energization of the relay SCR, the sparkout clutch 4TR, and therelay 6CR. A contact 6CR1 is closed which energizes a solenoid SOL C andthe piston rod 29 advances the work carrier 24 and the workpiece W intothe grinding position 30. A limit switch contact lLSA is closed whichenergizes a -carrier in limitrelay 4CR when the workpiece W is in itsinnermost position 30 and opens the limit switch contact lLSB whichdeenergizes the 'relay 6CR. A contact 4CR1 is closed to complete a cir-6CR1 opens to deenergize the solenoid SOL C and the piston rod 29retracts the work carrier 24.

The work carrier 24 and the workpiece W are reciprocated within apredetermined range as determined by reversal dogs (not shown) on thecarrier 24 by actuation of the limit switch contacts lLSA and lLSB, of alimit switch lLS, and a limit switch 2LS, to provide an improved surfaceon the sides of the workpiece W during the grinding operation.

The abrasive discs 14L and 14R continue advancing toward each otherwhile the work carrier 24 continues to reciprocate until the facecutinfeed limit switches 3LS and 4LS are closed, and facecut infeed isterminated.

The closing of the limit switches 3LS and 4LS complete a circuit throughthe contact 4TR1 to energize a sparkout motor 4TR. Continuedenergization of the sparkout motor 4TR ultimately closes a timed contact4TR4, which energizes an -air injectortimer relay 6TR, closing a -timedelaycontact 6TR1 which energizes an -air injectorrelay 10CR. A contact10CRl is closed which energizes a solenoid SOL F, and the valve 38 (FIG.2) is shifted to the right.

An un-metered volume of air is now directed from the line 36, throughthe valve 38, and through the lines 39 and 46 into the hollow spindle13R to supplement the continuous metered air flow already therein. Thevolume of air through the valve 38 is directed into the coolant with asufficient force and suddenness to eliminate the tendency for anymomentary vacuum to be formed between the abrasive discs 14L and 14R asthey are being separated rapidly. Therefore, the abrasive discs 14L and14R can be retracted moreassuredly in unison when workpiece size isreached at the end of the sparkout" operation.

The timed contacts 4TR1, 4TR2 and 4TR3 are simultaneously opened whenthe timed contact 4TR4 is closed when the sparkout clutch 4TR times out,as shown in FIG. 4. The opening of the contact 4TR1 deenergizes thesparkout motor 4TR.The opening of the contact 4TR2 deenergizes the relay7CR, and the contact 7CR1 opens to deenergize the solenoid SOL D, whichreverses the hydraulic pressure status in the facecut feed mechanisms10L andlOR. The opening of the contact 4TR3 deenergizes the relay SCR.The contact SCRl opens to deenergize the sparkout clutch 4TR and therelay 6CR which permits the sparkout clutch 4TR to reset. The contact6CR1 opens to deenergize the solenoid SOL C to efi'ect retraction of thework carrier 24 and the workpiece W from between the abrasive discs 14Land 14R, which are being retracted.

The abrasive discs 14L and 14R are retracted by the feed mechanisms 10Land 10R at a rapid rate. The

work carrier 24 and the workpiece W are then retracted. The solenoid SOLF is deenergized when the timer relay contact 6TR1 opens, after a timedelay following retraction of the abrasive discs 14L and 14R which areretracted in unison. The valve 38 is then returned to its originalposition by spring pressure and the un-metered air supply is blockeduntil the above opera tion is repeated for the next workpiece W.

It is to be understood that the air of supply line 37 could be connecteddirectly to one of the abrasive discs 14L or 14R rather than into one ofthe coolant lines. Also, it is not essential that air be supplied duringthe entire grinding operation. It is important, however, that air besupplied at a point in time which is early enough to prevent grinding toan undersize.

It is also to be understood that only a preferred embodiment of theinvention has been specifically illustrated and described, andvariations may be made thereto without departing from the invention, asdefined in the appended claims. I claim:

1. A double disc grinding machine for abrading the parallel sides of aworkpiece mounted on a workholder comprising a pair of abrasive discseach mounted on a rotatable spindle,

means for mounting said abrasive discs in coaxial spaced relation,

means for rotating said discs,

means for infeeding said discs to effect stock removal,

a source of liquid coolant,

means for directing the liquid coolant to at least one of the faces ofsaid abrasive discs for cooling the workpiece during stock removal,

a source of pressurized air, and

means for selectively introducing a controlled amount of air from saidair source into the liquid coolant within said directing means during astock removal to substantially maintain the equality of the hydrodynamicforces acting on opposing sides of the wo rkholder.

2. A double disc grinding machine according to claim 1, furthercomprising means for preventing the passage of liquid coolant from saiddelivering means to said air source.

3. A double disc grinding machine according to claim 1, furthercomprising means for selectively introducing a volume of air into saiddirecting means with sufficient force and suddenness at the conclusionof stock removal and prior to abrasive disc retraction to eliminate anyvacuum existent between the abrasive discs.

1. A double disc grinding machine for abrading the parallel sides of aworkpiece mounted on a workholder comprising a pair of abrasive discseach mounted on a rotatable spindle, means for mounting said abrasivediscs in coaxial spaced relation, means for rotating said discs, meansfor infeeding said discs to effect stock removal, a source of liquidcoolant, means for directing the liquid coolant to at least one of thefaces of said abrasive discs for cooling the workpiece during stockremoval, a source of pressurized air, and means for selectivelyintroducing a controlled amount of air from said air source into theliquid coolant within said directing means during a stock removal tosubstantially maintain the equality of the hydrodynamic forces acting onopposing sides of the workholder.
 2. A double disc grinding machineaccording to claim 1, further comprising means for preventing thepassage of liquid coolant from said delivering means to said air source.3. A double disc grinding machine according to claim 1, furthercomprising means for selectively introducing a volume of air into saiddirecting means with sufficient force and suddenness at the conclusionof stock removal and prior to abrasive disc retraction to eliminate anyvacuum existent between the abrasive discs.